Question

An aqueous solution of methyl alcohol (molar mass = 32.05 g/mol) is prepared by dissolving 15.0 mL of the alcohol (d = 0.792 g/mL) in 75.0 mL of water (1.00 g/mL).

a. Calculate the mass % of the aqueous alcohol solution.
b. Calculate the molarity of the aqueous alcohol solution.
c. If 5.0 mL of the solution (above) is diluted to a volume of 50.0 mL, what is the molarity (M) of the resulting dilute solution?

Answer 1

A. The mass% of the aqueous alcohol solution is 13.69%(wt/wt). The solution to this is:

mass alcohol = 15.0mL (0.792 g/mL) = 11.9 grams alcohol
mass water = 75.0mL (1.00 g/mL) = 75.0 grams water
total mass = 11.9 + 75.0 = 86.9 grams solution
mass% of methanol = (11.9/86.9)*100% = 13.69%

B. Molarity = 4.125 M

Solution:

Molarity = mole solute/L solution
mole solute = 11.9/32.05 = 0.3713 moles
L solution = 15 + 75 = 90 mL/1000 = 0.09 L

M = 0.3713/0.09 = 4.125

C. Answer is 0.4125 M

Solution:

C1V1 = C2V2
4.125(5mL) = C2(50mL)
C2 = 0.4125 M